1. Field of the Invention
The invention relates to a display device such as a display device including a plasma display panel or a digital micro-mirror device, and a method of displaying images in such a display device.
2. Description of the Related Art
Hereinbelow is explained how an image signal is processed in a plasma display panel as a typical example of a digital display device.
FIG. 1 is a block diagram showing how an image signal is processed in a conventional plasma display panel.
The illustrated plasma display panel is comprised of a first block 62 which receives an image signal 61, and applies inverse-gamma process to the received image signal 61, a second block 63 which receives an output signal transmitted from the first block 62, and carries out error diffusion, that is, spatially diffuses gray scales, a third block 64 which receives an output signal transmitted from the second block 63, and calculates an average picture level (APL), a fourth block 65 which receives an output signal transmitted from the third block 64, and converts the received output signal into sub-field (SF) codes, a frame memory 66 which receives an output signal transmitted from the fourth block 65, and outputs an image signal 69, and a fifth block 68 which receives the average picture level 67 from the third block 64, and outputs a sustaining pulse signal 70.
Hereinbelow is explained an operation of the plasma display panel illustrated in FIG. 1.
The first block 62 non-linearly converts the received image signal 61 in association with a gray scale such that the image signal 61 which was made on the assumption that images defined by the image signal 61 were displayed on a cathode ray tube (CRT) is suitable for being displayed in a plasma display panel.
For instance, the image signal 61 is input into the first block 62 as a signal having eight-bit gray scale for each of red (R), green (G) and blue (B), and then, non-linear conversion is applied to the image signal 61 in the first block 62 in accordance with the equation (A).y=x2.2  (A)
The first block 62 transmits an output signal having bits or the number of gray scales greater than the same of the image signal 61. On receipt of 8-bit R, G and B signals, the first block 62 generally outputs a 10-bit signal.
The second block 63 receives a signal transmitted from the first block 62. If the first block 62 transmits a 10-bit signal, for instance, the second block 63 spatially diffuses the lowest two bits among 10-bit gray scale resolution, and thus, outputs an 8-bit image signal to the third block 64.
On receipt of the image signal from the second block 63, the third block 64 transmits the received image signal to the fourth block 65 without applying any process to the image signal, and further, calculates an average picture level 67 of images defined by the received image signal.
The average picture level 67 calculated by the third block 64 is transmitted to the fifth block 68. The fifth block 68 converts the average picture level 67 into the number of sustaining pulses in dependence on which a luminance of images is determined, and transmits the number of sustaining pulses to a plasma display panel (not illustrated) as a sustaining pulse output signal 70.
The image signal transmitted to the fourth block 65 from the third block 64 is converted into sub-field coding data in the fourth block 65. A plasma display panel displays images at a certain gray scale defined by the sub-field coding data.
For instance, the fourth block 65 in a general plasma display panel converts an 8-bit image signal into 12 sub-field coding data.
The sub-field coding data is converted into an image output signal 69, and then, transmitted to the plasma display panel through the frame memory 66.
On receipt of the image output signal 69 from the frame memory 66 and further the sustaining pulse output signal 70 from the fifth block 68, the plasma display panel determines both which pixel is to be turned on or off and an intensity of light emission in pixels turned on, based on the signals 69 and 70, thereby displaying images.
Hereinbelow is explained a sub-field process to be carried out in the above-mentioned plasma display panel.
Herein, a sub-field process is a process in which a plurality of binary weighted pictures is overlapped one another time-wise to thereby display moving pictures having an intermediate gray scale.
As illustrated in FIG. 2, there is assumed a plasma display panel having pixels arranged in horizontally ten rows and vertically four columns. It is also assumed that a luminance for red, green and blue is displayed in 8-bit in each of the pixels, and that it is possible to display images at a luminance in 256 gray scales. Hereinbelow is explained a green (G) signal as an example of R, G and B signals.
In FIG. 2, an area A has a signal level of 128 luminance. In other words, a signal of (1000 0000) level is applied to each of pixels in the area A, if the luminance is expressed in a binary code. An area B has a signal level of 127 luminance. That is, a signal of (0111 1111) level is applied to each of pixels in the area B. An area C has a signal level of 126 luminance. That is, a signal of (0111 1110) level is applied to each of pixels in the area C. An area D has a signal level of 125 luminance. That is, a signal of (0111 1101) level is applied to each of pixels in the area D. An area E has a signal level of 0 luminance. That is, a signal of (0000 0000) level is applied to each of pixels in the area E.
Herein, it is assumed that 8-bit signals in each of the pixels are arranged along a time axis in a spatial position of each of the pixels. A sub-field is defined as X/8 wherein X indicates a period of time in which images in a frame are displayed. In other words, in a method of displaying images in accordance with a sub-field process in which a frame or field is divided into a plurality of differently weighted binary images, and the binary images are overlapped one another time-wise to thereby display images, a binary image divided from a frame is defined as a sub-field.
Since each of pixels has 8 bits, one field is divided into first to eighth subfields SF1 to SF8, as illustrated in FIG. 3.
As illustrated in FIGS. 4A to 4H, the first sub-field SF1 is comprised of the lowermost bits in 8-bit signals in each of pixels, arranged in a 10×4 matrix. Similarly, the second sub-field SF2 is comprised of the second lowermost bits in 8-bit signals in each of pixels, arranged in a 10×4 matrix. The third to eighth sub-fields SF3 to SF8 is comprised of bit in the same way as the first or second sub-field SF1 or SF2.
FIG. 5 illustrates plasma display panel drive signals for one field.
As illustrated in FIG. 5, the first to eighth sub-fields SF1 to SF8 are processed in this order in one field.
Hereinbelow is explained how each of the sub-fields is processed, with reference to FIG. 5.
Each of the sub-fields is comprised of a set-up period P1, a writing period P2 and a sustaining period P3.
In the set-up period P1, a pulse is singly applied to a sustaining electrode and a scanning electrode. As a result, preliminary discharge is generated.
In the wiring period P2, scanning electrodes arranged in a horizontal row is scanned in sequence, and writing is carried out only to pixels which received a pulse from a data electrode. For instance, while the first sub-field SF1 is being processed, writing is carried out to pixels indicates as “1”, and writing is not carried out to pixels indicated as “0” in the first sub-field SF1 illustrated in FIG. 3.
In the sustaining period P3, a sustaining pulse (a driving pulse) is output to each of the sub-fields in accordance with weighting. In a pixel indicated as “1”, that is, to which writing has been carried out, plasma discharge is generated in response to the application of a sustaining pulse thereto. One plasma discharge gives certain brightness to a pixel. Since the first sub-field SF1 is weighted one, there is obtained a brightness of level one. Since the second sub-field SF2 is weighted two, there is obtained a brightness of level two.
As is obvious, the writing period P2 means a period in which a pixel or pixels from which a light is emitted is(are) selected, and the sustaining period P3 means a period in which a light is emitted by the number associated with weighting.
As illustrated in FIG. 5, the first to eighth sub-fields SF1 to SF8 are weighted 1, 2, 4, 8, 16, 32, 64 and 128, respectively. Accordingly, a brightness in each of the pixels can be varied in 256 steps from 0 to 255 (1+2+4+8+16+32 +64+128=255).
In the area B illustrated in FIG. 2, a light is emitted from the selected pixels in the first to seventh sub-fields SF1 to SF7, and a light is not emitted in the eighth sub-field SF8. Accordingly, there can be obtained a brightness at 127 level (1+2+4+8+16+32+64=127).
In the area A illustrated in FIG. 2, a light is not emitted in the first to seventh sub-fields SF1 to SF7, and a light is emitted from the selected pixels in the eighth sub-field SF8. Accordingly, there can be obtained a brightness at 128 level.
The number of sub-fields and pseudo-framing noise are closely linked with each other. For instance, pseudo-framing noise can be reduced by increasing the number of sub-fields.
Hereinbelow is explained pseudo-framing noise.
As illustrated in FIG. 6, it is assumed that the areas A, B, C and D are shifted to the right by a distance equal to a width of a pixel in comparison with the arrangement illustrated in FIG. 2. Accordingly, a viewing point of a viewer moves to the right, following the areas A, B, C and D. With the areas A, B, C and D being shifted, three pixels vertically arranged in the area B (three pixels in the area B1 in FIG. 2) are replaced with three pixels vertically arranged in the area A (three pixels in the area A1 in FIG. 6) after one field is past.
When the image illustrated in FIG. 2 is changed into the image illustrated in FIG. 6, the binary data (01111111) in the area B1 in FIG. 2 and the binary data (10000000) in the area A1 in FIG. 6 are recognized by a viewer as data (00000000). That is, the area B1 is not displayed at its original 127 brightness level, but displayed at 0 brightness level. As a result, an apparent dark framing line appears in the area B1.
As mentioned above, when an upper bit is apparently changed to “0” from “1”, there appears an apparent dark framing line.
In contrast, when the image illustrated in FIG. 6 is changed to the image illustrated in FIG. 2, a viewer recognizes the area A1 as having data (11111111), based on the binary data (10000000) of the area A1 and the binary data (01111111) of the area B1. That is, this means that an uppermost bit is compulsorily changed to “1” from “0”, and hence, the area A1 is not displayed at its original 128 brightness level, but displayed at 255 brightness level about twice greater than 128 brightness level. As a result, an apparent bright framing line appears in the area A1.
As mentioned above, when an upper bit is apparently changed to “1” from “0”, there appears an apparent bright framing line.
A mechanism of generation of pseudo-framing noise in a plasma display panel is described in detail in Uchiike et al., “All about Plasma Display”, Kogyo-Chousakai, pp. 163–177, for instance.
A framing line appearing in a display screen is called pseudo-framing noise only with respect to moving pictures. Pseudo-framing noise deteriorates display quality.
In general, the number of sub-fields to be displayed in a frame or field in a display device such as a plasma display device or a digital micro-mirror device is dependent on characteristics of the display device. For instance, the number of sub-fields in a plasma display device is generally eleven or twelve. Images are displayed in accordance with the number of sub-fields determined in each of display devices. In order to enhance display quality, there are two methods, in one of which a gray scale control is emphasized, and in the other of which reduction in pseudo-framing noise is emphasized.
In accordance with the former method, it would be possible to display images at 12-bit gray scale in a plasma display panel which can display images in 12 sub-fields, for instance. In accordance with the latter method, it would be possible to display images at 8-bit gray scale, and apply the remainder 4 bits to redundancy coding for the purpose of reducing pseudo-framing noise. Redundancy coding is used generally for reduction in pseudo-framing noise.
As an example of the former method, a method of displaying images, suggested in Japanese Patent Application Publication No. 6-259034 is explained hereinbelow. As an example of the latter method, a display device suggested in Japanese Patent No. 2994630 (Japanese Patent Application Publication No. 11-231825) is also explained hereinbelow.
FIG. 7A is a block diagram of an apparatus for carrying out the method suggested in Japanese Patent Application Publication No. 6-259034.
The illustrated apparatus is comprised of a first circuit 71 for applying gamma-compensation to and changing a level of R, G and B video signals, a field memory 72 electrically connected in series to an output of the first circuit 71, a plasma display panel driver 73, a plasma display panel 74, an integration circuit 75 which receives a luminance signal Y generated based on the R, G and B video signals, and integrating the luminance signal Y to thereby output an average picture level (APL), a control circuit 76 which receives the average picture level (APL) from the integration circuit 75, compares the received average picture level to a predetermined level to thereby group a brightness of images into three levels, transmits a control signal associated with each of the three levels, to a later mentioned second circuit 77, groups each of the levels into three sub-levels, and transmits a control signal associated with each of the three sub-levels, to the first circuit 71, a second circuit 77, and a display control circuit 80.
The second circuit 77 is comprised of a first counter 78 for counting the number of sub-fields, and a second counter 79 for counting the number of display pulses. The second circuit 77 transmits a display timing pulse to the display control circuit 80 at a predetermined timing in accordance with the control signal received from the control circuit 76.
In the display device illustrated in FIG. 7A, a field display period for each of pixels is time-divided into sub-field periods having N-bit gray scales, and the number of display pulses in each of the sub-field periods is weighted for displaying images at intermediate gray scales.
Specifically, the control circuit 76 selects the number of gray scale bits in accordance with a brightness level of displayed images such that the brighter displayed images are, the greater the number of display gray scales is. If the average picture level is smaller than 10%, as shown with the pattern {circle around (1)} in FIG. 7B, a 8-bit gray scale signal having the maximum number of display pulses of 512 is level-changed into a signal having the maximum number of display pulses of 896, and if the average picture level is equal to or greater than 10%, but smaller than 25%, as shown with the pattern {circle around (2)} in FIG. 7B, a 8-bit gray scale signal having the maximum number of display pulses of 512 is changed in level into a signal having the maximum number of display pulses of 640.
In accordance with the display device illustrated in FIG. 7A, the number N of sub-fields is switched into a smaller one, and hence, the number of addressing periods is reduced, in a dark scene in which an average picture level is low. As a result, a maximum of a display luminance is not reduced even in a dark scene, and hence, a contrast ratio is not reduced. In the display device illustrated in FIG. 7A, the number of sub-fields is made smaller for images having a smaller average picture level or darker images, to thereby make a maximum of display gray scale greater, and a gray scale of an output signal transmitted from the first circuit 71 is arbitrarily changed, ensuring that images are displayed at gray scales with high quality.
FIG. 8 is a block diagram of a display device suggested in Japanese Patent No. 2994630.
The illustrated display device is comprised of a first circuit 81 which receives a vertical synchronization signal and a horizontal synchronization signal, and outputs a timing pulse signal, an analog-digital (A/D) converter 82 which converts analog R, G and B signals into digital R, G and B signals, a first device 83 which applies inverse-gamma compensation to the analog-digital converted R, G and B signals, a second device 84 which delays the R, G and B signals to which the inverse-gamma compensation has been applied, by one field, a multiplier 85 which receives the R, G and B signals having been delayed by a field, and a later mentioned constant-multiplication coefficient A, and multiplies by them each other, a peak level detector 93 which detects a brightest peak in a field, an average level detector 92 which calculates an average of a brightness in a field, a third device 94 which receives a peak level signal transmitted from the peak level detector 93 and an average level signal transmitted from the average level detector 92, and determines four parameters (a weighting number N, a constant-multiplication coefficient A of the multiplier 85, the number Z of sub-fields, the number K of gray scale display points), based on a combination of the signals, a fourth device 86 which receives the number K of gray scale display points from the third device 94, and converts a brightness signal expressed in a certain fineness, into a gray scale display point closest to the brightness signal, a fifth device 87 which receives the number Z of sub-fields and the number K of gray scale display points from the third device 94, and converts a 8-bit signal transmitted from the fourth device 86, into a Z-bit signal, a sixth device 95 which receives the weighting number N, the number Z of sub-fields and the number K of gray scale display points from the third device 94, and determines the number of sustaining pulses necessary for each of sub-fields, a seventh device 88 which determines the number of sustaining pulses to be transmitted in a sustaining period P3, in accordance with a signal transmitted from the sixth device 95, a detector 96 which detects a vertical synchronization frequency, a second circuit 89 for driving data electrodes, a third circuit 90 for driving scanning and sustaining electrodes, and a plasma display panel (PDP) 91.
In the display device illustrated in FIG. 8, for instance, when the average level detector 92 detects a high average level, the number Z of sub-fields is increased and the weighting number N is reduced for preventing an increase in both power consumption and a temperature of the plasma display panel 91. It would be also possible to reduce a pseudo-framing line by increasing the number Z of sub-fields.
When the average level detector 92 detects a low average level, the number Z of sub-fields is reduced, and the number of writing in a field is also reduced. Time obtained by reducing the numbers can be used for increasing the weighting number N. Accordingly, it would be possible to display images brightly even in darkness.
As mentioned earlier, the display device illustrated in FIG. 7A makes great account of enhancement in gray scale characteristics, and accordingly, does not always deal with the pseudo-framing line problem.
In contrast, the display device illustrated in FIG. 8 makes great account of reduction in a pseudo-framing line, and does not make particular attempt to enhance gray scale characteristics.
Herein, there is considered a scene having a low average picture level, for instance, a scene in which a crow flies in the night darkness under a full moon.
In accordance with the display device illustrated in FIG. 7A, a moon which can raise a maximum luminance is displayed at a high luminance, and it would be possible to display the scene at a high contrast. However, since the display device illustrated in FIG. 7A increases the number of gray scales and concurrently reduces the number of sub-fields, a pseudo-framing line significantly deteriorates images displayed.
In accordance with the display device illustrated in FIG. 8, a moon can be displayed at a high luminance, since it would be possible to raise a maximum luminance by increasing the weighting number N, ensuring it possible to display the scene at a high contrast. However, the display device illustrated in FIG. 8 reduces the number Z of sub-field in displaying images, and hence, images are deteriorated by a pseudo-framing line. In addition, since the number of gray scales is kept fixed, it would be difficult to distinguish a crow and the night darkness from each other in comparison with the display device illustrated in FIG. 7A.
Hence, an object of the present invention is to make it possible to distinguish a crow and the night darkness from each other, and prevent pseudo-framing characteristic from deteriorating, even in displaying images having a low average picture level.
Analyzing various pictures in TV programs or movies, the inventor had found the following fact.
In a scene having a low average picture level, it is necessary in a dark area to increase the number of gray scales for distinguishing slight differences among gray scales from one another. In images displayed in a plasma display panel, a viewer can scarcely find a pseudo-framing line, even if images are moving. In a scene having a low average picture level, a pseudo-framing line is sometimes conspicuous, if images displayed in a bright area move. However, in a scene having a low average picture level, such images displayed in a bright area scarcely move in such a speed that a pseudo-framing line is conspicuous.
Herein, there is considered again a scene in which a crow flies in the night darkness under a full moon.
It is assumed in the scene that a moon moves at such a speed that a pseudo-framing line is conspicuous and a viewer can follow the moon with his/her eyes. In the display device illustrated in FIG. 7A, a pseudo-framing line would be remarkably conspicuous around the moon. In the display device illustrated in FIG. 8, it would not be possible to distinguish the crow and the night darkness from each other.
Japanese Patent Application Publication No. 8-23460 has suggested a circuit for carrying out dynamic gamma-compensation, including first means for dividing image level of an input signal into a plurality of sub-levels, second means for calculating a degree in each of the sub-levels, and third means for grouping the degrees of each of the sub-levels into a plurality of levels.
Japanese Patent Application Publication No. 2001-282183 has suggested a gray scale controller in a plasma display panel, including a detecting circuit which monitors M-bit digital video signals in N frames, checks whether a bit is vacant in the monitored frame in an order from an uppermost bit to a lowermost bit, and transmits an output bit selecting signal associated with a vacant bit and a table switching signal, a selector which outputs bits from which vacant bits are removed from M bits and which are arranged sequentially from an uppermost bit to a lower bit, the bits being smaller than M bits and being output in accordance with the output bit selecting signal, a memory storing a plurality of tables used for determining a weighting for each of sub-frames in the plasma display panel, the tables being switched in accordance with the table switching signal, and an interface which makes access to the memory, controls a sustaining pulse in each of the sub-frames, and transmits a light-emission pattern to a driver in a next stage.